Tuning Rel G1

Right now crossover is at 35 db. And volume level is 6. Some tracks have decent bass but other are full of base. Which make me either skip these tracks or I have to lower the base, I do not want to adjust the base for every other track.

I totally agree with your recommendations. I did bring it down to 30 HZ but not to 25 HZ and the reasons are as follows,
I have a Home theater 7.1 system which is as follows

B & W 802D Front Speakers
Frequency range -6dB at 27Hz and 33kHz
Frequency response 34Hz – 28kHz ±3dB on reference axis

Center
B & W HTM2 D
Frequency range -6dB at 35Hz and 33kHz
Frequency response 41Hz - 28kHz ±3dB on reference axis

Side Speakers
Monitor Audio PMC 702 Reference
Frequency response 35HZ - 20KHz

Back
Mirage MX
Frequency Response (Hz, +/-3dB) 110 Hz - 20 kHz (+/- 3dB)

Now on my processor, I cannot lower down crossover frequency below 40Hz.
Just clarify, front speakers are set to full range for base and not "Full + Sub". I have a separate connection from processor to .1/LFE input.
Now front speakers have a frequency response from 34 Hz and up and that's why I set it to 35 HZ.
My questions are
1) If I set my sub-woofer to 25 HZ, than what happens to frequency from 25HZ to 35Hz which cannot be played by these speakers.
2) Several people commented that by adding this sub-woofer they got a wider sound stage. If I drop the crossover frequency to 25 HZ, how someone can have a wider sound stage, I am not very knowledgeable in this field but I think when someone says wider sound stage that means some of the mid-range frequencies are played through the sub-woofer. But will they at 25db?
3) For surround sound (7.1), if my processor has a lowest crossover frequency response of 40HZ and I set my sub-woofer to anywhere below 40Hz, what happens to the difference?

All these questions are because I paid $4500 for the sub and another $2500 for two cables one High level input and one for .1/LFE, so I want to do it right and I don't know “what is” and “who to” make it right.

Okay I will try this weekend to drop it to 25HZ and see what happens.

I dropped the Crossover from 35 to 25 and then even to 20 hz and started to increase it little by little. At 20 to 25 Hz I had to increase the volume from 6 to 9 but after 25 I had to go back to 6 otherwise the bass was too high. I also spoke to my dealer who sold me the subwoofer, his was same as I stated in my conclusion. Also last week I had Audio Professional to tune my system for room correction.
Conclusion: As everybody on the form said, volume number does not matter. Adjust it where it sounds right. Right now my sub is still set for volume number 6 and crossover of 40HZ. At 40 Hz sound stage does improve as compared to at 25Hz. There are some songs which have more bass than the others but overall the result is pretty good.